The overall objective of this research is to further our understanding of certain organizational (design) and functional features of the segmental motor control system. Fifteen experiments are proposed to explore the spinal and peripheral mechanisms that govern the interactions that occur between motor units and muscle receptors during the development of muscle force. This approach should contribute considerable insight into the still unresolved issue of the full role played by muscle spindles and tendon organs in motor control. It is suggested that the fundamental unit of the motor nucleusmuscle complex is the motoneuron-muscle unit-muscle receptor-motoneuron loop and that the "sensory partitioning" that is known to occur within the muscle itself has a parallel in "synaptic partitioning" within the homonymous motoneuron pool of the spinal cord. The main peripheral aspects of spindle and tendon organ function that we wish to test involve their interactions with motor units in an active rather than passive muscle. At the spinal level our principal concern lies in testing whether the strength of mechanical coupling between a motor unit and a given muscle receptor is correlated with the strength of synaptic coupling between the motoneuron innervating that motor unit and the afferent fiber innervating that muscle receptor. It is hoped that these studies will contribute information on a fundamental issue in motor control that has yet to be resolved. Furthermore it bears emphasis that many neuromuscular diseases involve degeneration of alpha axons with subsequent changes in the innervation ratio and territory of motor units (i.e., muscle unit "clumping"). Such changes have implications for motor unit-muscle receptor interactions at both the peripheral and central level of the nervous system. For this reason our studies will hopefully contribute to our understanding of diseases such as ALS, adult spinal muscular atrophy, Friedreich's ataxia, and those of Werdnig-Hoffman, Kugelberg-Welander and Charcot-Marie Tooth.